In recent years, attempts have been made to reduce exhaust gases such as CO2 from the viewpoint of global environment conservation. In the automobile industry, measures have been taken to reduce the amount of exhaust gases by increasing fuel efficiency through the weight reduction of an automobile body.
Examples of a method for reducing the weight of an automobile body include a method in which the thickness of a cold-rolled steel sheet used for an automobile is decreased by increasing the strength of the steel sheet. However, since it is known that there is a problem with this method in that bendability decreases with an increase in the strength of a cold-rolled steel sheet, there is a demand for a cold-rolled steel sheet having a high strength and satisfactory bendability at the same time. There is a tendency for a variation in mechanical properties within a high-strength cold-rolled steel sheet to increase with an increase in the strength level of the cold-rolled steel sheet. Therefore, there is a demand for an increase in the stability of bendability within a cold-rolled steel sheet from the viewpoint of increasing material yield in the case where a part is manufactured by performing form forming which involves many portions to be subjected to bending. Here, generally, it is possible to use the ratio of a limit bending radius to a thickness (R/t) as an index for evaluating the stability of bendability, and it is possible to judge that the smaller the value of R/t is, the more stable the bendability within a cold-rolled steel sheet is.
In response to the requirement described above, for example, Patent Literature 1 discloses a high-strength cold-rolled steel sheet having a tensile strength of 780 MPa to 1470 MPa, good shape, and excellent bendability and a method for manufacturing the steel sheet. When a steel sheet having a chemical composition within a specified range is reheated after overcooling has been performed without stopping cooling at a specified bainite transformation temperature, tempered martensite is partially mixed into a microstructure or various kinds of bainite different in hardness from each other exist as a result of transformation occurring at different temperatures. Even in such a case, Patent Literature 1 discloses that, when the volume fraction of a retained austenite phase having an Ms transformation temperature of −196° C. or higher is 2% or less, there is practically no decrease in bendability compared with a case where cooling is stopped at a specified bainite transformation temperature, and there is a significant improvement in shape compared with the case where cooling is first performed to room temperature and reheating is then performed. Although bendability is evaluated by performing a 90-degree-bending test, since no consideration is given to a position to be evaluated, the stability of bendability is not disclosed.
Patent Literature 2 discloses a steel sheet excellent in bendability and drilling resistance. Patent Literature 2 discloses a method in which bendability is increased, for example, by rapidly cooling a steel sheet after rolling has been performed or after rolling followed by reheating has been performed in order to form a microstructure including mainly martensite or a mixed microstructure including martensite and lower bainite and by controlling the value of Mn/C to be constant over the full range of the C content. Although bendability is evaluated by using a press bending method, since no consideration is given to a position to be evaluated, the stability of bendability is not disclosed. Moreover, although specification regarding Brinell hardness is disclosed, specification regarding tensile strength is not disclosed.
Patent Literature 3 discloses a high-strength steel sheet excellent in bendability and a method for manufacturing the steel sheet. Patent Literature 3 discloses a method in which a steel sheet having good close-contact bending capability in any one of the rolling direction, the width direction, and the 45-degree direction is manufactured by heating steel having a specified chemical composition, performing rough rolling, performing hot finish rolling which is started at a temperature of 1050° C. or lower and finished in a temperature range from the Ar3 transformation temperature to (the Ar3 transformation temperature+100° C.), cooling the hot-rolled steel sheet at a cooling rate of 20° C./s or less, coiling the cooled steel sheet at a temperature of 600° C. or higher, performing pickling, performing cold rolling with a rolling reduction of 50% to 70%, performing annealing for 30 seconds to 90 seconds in a temperature range in which an (α+γ)-dual phase is formed, and cooling the annealed steel sheet to a temperature of 550° C. at a cooling rate of 5° C./s or more. Although bendability is evaluated by performing close-contact bending, since no consideration is given to a position to be evaluated, the stability of bendability is not disclosed. Moreover, although tensile properties are evaluated by performing a tensile test, since the steel sheet has a strength of 980 MPa or less, the steel sheet has insufficient strength to be used as a high-strength steel sheet for an automobile.